what is instancing in blender
There is a bunch of definitions for Instance. The article you mention use the definition: Make it duplicate when in rendering pipeline
Instead of Alt+D, instancing will not likely directly been manipulated by user. E.g. you won't Alt+D for 1000 trees in a mountain, you will rather use particle system to plant instances for you, with some weight control or random method.
But if you want to place three identical boxes in specified places, you will want to Alt+D for actually manipulate the position for boxes.
They are similar concept but with different strategy. Depends on how you want to use them.
Other Instances
Instance is a common computer terminology that indicate we make something actually exist in memory and scene or workspace.
In Unity, instantiating an object mean to make a copy from its resources. Which might be totally as same as Alt+D in Blender. In Python or other OOP, all objects you manipulate are an instance from their class. Also, instance might mean to separate some minimum attribute to make it more easily to manipulate without having a full copy.